Wind and solar energy have seen significant decreases in the cost of these technologies over that last decade which has lead to increasing levels integrated into the grid. They also offer unique benefits such as no fuel costs, quick installation, and no pollution. But as more variable renewable energy (VRE) such as wind and solar is integrated into electrical power systems, a range of new technical challenges is becoming more prevalent in planning and operating the grid. Challenges include dealing with variable and uncertain power production as well as the unique physical characteristics that wind and solar generators have. This special section looks at some of the new techniques to solve these challenges with integrating ultra-high levels of VRE. ?
In the paper entitled “Integrating high levels of variable renewable energy into electric power systems”, the challenges and solutions to integrating ultra-high levels of variable renewable energy into electric power systems are reviewed. A summary of power system operations is given to better understand different aspects and how they can be affected by increasing levels of wind and solar energy. Then specific technical challenges and mitigation measures are described. Finally, several examples of operating power systems with a wide range of VRE are discussed.
In the paper entitled “Double-uncertainty optimal operation of hybrid AC/DC microgrids with high proportion of intermittent energy sources”, the authors introduce double-uncertainty optimization theory into the optimization operation in the AC/DC hybrid microgrid to solve the uncertainty problem caused by high levels of variable renewable energy sources. In the day-ahead scheduling stage, a fuzzy stochastic expectation economic scheduling model based on the uncertain optimization theory is proposed to minimize the comprehensive operation costs of hybrid AC/DC microgrid. In the intraday adjustment stage, a real-time unbalanced power adjustment model is established to minimize real-time adjustment costs. Through comparative analysis of deterministic optimization, stochastic optimization and fuzzy stochastic optimization in the day-ahead scheduling stage and the real-time adjustment stage, the validity of fuzzy stochastic optimization based on fuzzy stochastic expectation model is proved in solving hybrid AC/DC microgrid scheduling problem.
In the paper entitled “District heating system operation in power systems with high share of wind power”, a short-term operation model of a district heating system is proposed to optimally schedule the production of both heat and power in a system with high wind power penetration. The application of the model in a case study system shows the increased flexibility offered by the coordination of power generation, consumption and heat storage units, which are available in district heating systems.
In the paper entitled “Complete active-reactive power resource scheduling of smart distribution system with high penetration of distributed energy resources”, an economic framework based on the active-reactive power bids is developed for complete active-reactive power dispatch scheduling of smart distribution networks. It will be essential to schedule active-reactive power at the distribution level in future power systems because of the high number of distributed energy resources (DER). The economically complete active-reactive power scheduling approach suggested in this allows both active power markets and the volt/var control schemes to cooperate. Using DER’s reactive power capability, a generic framework of reactive power offers for DERs is implemented on a 22-bus distribution test system to verify the suggested active-reactive power scheduling approach.
In the paper entitled “Security-constrained line loss minimization in distribution systems with high penetration of renewable energy using UPFC”, the authors propose a novel control scheme using a unified power flow controller (UPFC) to achieve line loss reduction and security enhancement in distribution systems with high penetration of renewable energy. The line loss minimum conditions of a general distribution system with loop configurations are deduced. Then security constraints including the permissible voltage range, the line loading limits and the UPFC ratings are considered. System security enhancement at the least cost of the line loss increase is tackled by solving a reduced optimal power flow (OPF) problem. Line loss minimization is achieved through a dynamic controller, while an OPF calculator is integrated to generate corrective action to the dynamic controller when the security constraints are violated. Finally, the validity of the proposed control strategies is then verified in a modified IEEE 33 bus test system.
In the paper entitled “Transient stability enhancement of DC-connected DFIG and its converter system using fault protective device”, the authors study an overcurrent and overvoltage protective devices (OCV-PD) to ensure that DC-based doubly fed induction generators (DFIG) system can keep connected to the grid and operate correctly during the faults. This method is compared with series dynamic braking resistor (SDBR) and two aspects are improved. First, the two-level control strategy and dc inductor circuit are adopted to ensure that the OCV-PV could adapt to the variable power conditions and limits the current impulse to protect the DFIG system during the overcurrent fault. Second, it is shown through simulation that the OCV-PD can protect the system from overvoltage faults.
In the paper entitled “Consensus control of electric spring using back-to-back converter for voltage regulation with ultra-high renewable penetration”, the authors describe a new version of “electric springs” based on a back-to-back converter configuration that extends the operating range and improves the voltage suppression performance to facilitate ultra-high renewable penetrations. This paper proposes an efficient control method to facilitate the voltage regulation functions with non-critical load. Particularly, the proposed method is suitable to various loads with different characteristic. The authors also develop a consensus algorithm to coordinate multiple energy springs for maintaining critical bus voltage in distribution systems with ultra-high renewable penetration. The proposed operation method is verified on a modified IEEE 15-bus distribution network.
In the paper entitled “Optimized dispatch of wind farms with power control capability for power system restoration”, the authors discuss how wind farms can be used to contribute during system restoration. A robust optimization model is proposed that maximizes the output of the wind farms for participating in power system restoration while simultaneously ensuring the security of the restored systems accounting for any uncertainty in transient power sag. Simulation results demonstrate that the security constraints of the restored system can be kept within security limits with the power reference optimized by the proposed method.
We would like to thank all the participating authors for submitting their works to this special section. We would also like to thank the editorial staff at Journal of Modern Power Systems and Clean Energy and six guest editors for their assistance and effort in completing a quick turnaround of this special section.